This invention is directed to apparatus for displaying the position of vessels at sea and in particular to apparatus for controlling the display of data from a VHF direction finder on a conventional marine radar plan-position indicator (PPI).
Both radar and VHF direction finding have long been tools for fixing the position of vessels at sea from shore stations. The types of information supplied by these two systems are different. Whereas radar can accurately locate the position of a vessel with relation to shoreline features and other vessels, it does not provide a means for positive identification of vessels not carrying radar transponders. If there are two or more VHF direction finders in use, the position of a vessel can be determined while it is transmitting with its VHF communication equipment. Also, the vessel can be identified, if its transmissions are properly identified.
These techniques can complement one another, if the output of the systems can be correlated. Although this can be done on a plotting table, there is little time for this in a busy Vessel Traffic Management Centre and it is preferable to display the VHF/DF information directly on the radar display. Once this is done, VHF/DF can be used to extend the radar to areas where its coverage is poor or where signals are submerged in sea clutter, to bring an operator's attention to a vessel he might have otherwise missed.
The VHF/DF information is commonly indicated by numerically displaying the transmitter bearing, or, by having a distinctive symbol or a pointing vector appear on the display. The distinctive symbol is most commonly used in synthesized display systems and can consist of a cross, rectangle, circle or other symbol, while the pointing vector is a simple radial from the origin of the PPI, which terminates at the triangulated transmitter position. The latter is the easier to realize on a conventional radar PPI. However both suffer from the disadvantage that an error in either of the two VHF/DF bearings results in an error in the total display. Furthermore, if, say, the local VHF/DF were unserviceable, there would be no display at all.